I had uncapped internet and paid $43/mo ($53/mo for customers without cable TV).
For a 12Mbps line, which is reasonably fast, this isn't outrageous. $40/mo is fair.
I still fail to see how the amount of data used costs the Cable ISP any money. I'm assuming that what they do is merely divide up a direct connection to a backbone of some kind. Why on Earth they would have a metered line themselves is beyond me. And if it is unmetered, as it ought to be, then they don't particularly care how much data one customer transfers.
If they need to implement a system so that one person doesn't spoil the local node for everyone else, then do so my slowing down that one user (just their speed, thats it). Don't do it by capping everyone.